Slow listing projects for user with many role assignments

[Boris Bobrov]

With complex structure of inherited roles and groups listing projects and domains a user has access to becomes very slow. Some users are complaining that it takes a minute to get their list of roles.

Some time ago a similar bug has been fixed in https://bugs.launchpad.net/keystone/+bug/1700852, but it seems that there was no real fix and that we just added caching for the call.

It seems that there are several places where too many database calls are made:

1. For example, https://github.com/openstack/keystone/blob/ef711cd0eb5fb6fb143783ad4096e3f1dbd9da99/keystone/resource/core.py#L894 could be cached.

2. As mentioned in #1700852, _strip_domain_roles is called many times and is slow

3. add_implied_roles (https://github.com/openstack/keystone/blob/ef711cd0eb5fb6fb143783ad4096e3f1dbd9da99/keystone/assignment/core.py#L629) is slow, probably because `role_api.list_implied_roles` could be cached.